The automotive industry is served with a broad offering of light emitting diode (LED) lighting products for conventional 12 volt vehicular electrical systems. Some of these products include LED light strips, LED light pods, LED light bars, and LED light assemblies that are configured for a variety of uses. Many conventional LED light strips claim flexibility and are flexible on one plane. This allows them to be mounted on a surface with a mono-directional curve. Other types of automotive LED lighting products such as LED light pods and LED light bars are manufactured with rigid housings that encase the LEDs. Such housings include any number or configuration of LEDs.
Most of the products currently available on the market accept a supply voltage of 12 V DC. To protect the LEDs from overvoltage and thermal runaway, the LEDs in conventional products are placed on a printed circuit board in a series of three LEDs, followed by a resistor to control the current passing through the LEDs. This is necessary due to the very sensitive nature of LEDs and their susceptibility to thermal runaway and failure due to overheating.
The present disclosure contemplates that an LED is a diode and its voltage versus current characteristics are similar to other diodes in that the forward current is an exponential function of the forward voltage so that a small change in forward voltage can result in a large change in the forward current. For example, a conventional red 1206 LED may have an allowable DC forward current of 20 mA and a forward DC voltage of 1.9 V. However, if the forward voltage is increased to 2.1 V the forward current may increase to 50 mA, far exceeding the maximum allowable forward current for the LED. As the forward current increases beyond the maximum allowable limit, the LED will overheat causing the LED's voltage drop to decrease and causing more current to pass through the LED. This starts the cycle known as thermal runaway that quickly leads to the overheating and destruction of the LED. Traditionally, current limiting is the preferred method to control thermal runaway.
While conventional manufacturers have deemed this form of current control adequate, there are several concerns regarding this method, as follows:
1. The internal circuits of conventional systems have a fixed resistance chosen based on the belief that automotive systems operate on 12 V. The formula for calculating the resistance value is as follows:
  R  =            (                        V          S                -                  V          L                    )        I  where R is the resistor value, VS is the supply voltage, VL is the LED forward voltage, and I is the LED forward current. For example, if a device uses three red LEDs with a forward voltage of 1.9 V DC and a forward DC current of 20 mA in series with a supply voltage of 12 V, the circuit would require a 330 ohm resistor. However, automotive electrical systems do not maintain a constant 12V DC. Due to variations in loads, battery conditions, alternator output performance and other conditions, it is not uncommon to see variances from 8V DC to 16V DC and transient load dumps up to 120 V DC in conventional automotive systems. The supply voltage to the LED lighting device can vary widely due to functions such as turning headlights on and off, activating the starter, engaging and disengaging air conditioning compressors, engine RPMs, and other changes to the loading/charging of the electrical system. In the example given above, if the same circuit with three red LEDs were to receive a supply voltage of 8 V DC, the required resistor would be 120 ohms, which is a significant difference from the 330 ohm resistor that was chosen. The result would be dim and uneven lighting of the LEDs. In the same system, if the supply voltage was to be 16 V DC, the required resistor would be 560 ohms. However, since the chosen resistor was 330 ohms, the system would not have the needed current control and would be at risk for shortened LED life and/or thermal runaway.
2. The use of resistors (in conventional systems) internally to control current reduces the efficiency of the circuit in that the resistors take the excess current and turn it into heat. In many cases, hundreds and sometimes thousands of LEDs are used in accent lighting in the automotive applications. The increase draw on the automotive electrical systems can be detrimental. Many times accent lighting is turned on when the vehicle is not running and, therefore, the automobile battery is not being recharged. Increasing the current draw through the LED lighting devices by way of using resistors to limit current can shorten the span of time that the accent lights can be left on without draining the battery.
3. Conventional lighting devices have no protection against transient load dumping. Transient load dumps can be as high as 120 V DC and take up to 400 ms to decay. This kind of spike has been known to easily damage LEDs.
4. Conventionally, the need for internal regulation of current complicates the construction of such lighting devices and limits how small and discreet the devices can be made. The use of printed circuit boards (either flexible or rigid) also causes the devices to be opaque, and a need to hide the circuitry arises if the device is going to be in plain view. In the case of a flexible LED light strip that uses a conventional flexible printed circuit board internally, the strip is limited to flexing only on one plane because of the planer nature of the conventional flexible printed circuit board.
5. In the case of a conventional LED light strip that can be cut to length when a circuit of three LEDs and a resistor are used, the strip can only be cut in intervals of three LEDs. If the strip is cut anywhere within the three LEDs and resistor circuit, the circuit is broken and all three LEDs stop functioning.
Some conventional systems are directed to methods for arranging LEDs in parallel. However, they do not address a method for controlling thermal runaway or a condition known as load hogging or current hogging that occurs in parallel configurations of LEDs. When LEDs in conventional systems are arrayed in a parallel manner and the source current is limited to the total forward current of the string of LEDs, current hogging or load hogging occurs because of small variations in the LEDs. One or more LEDs will start to consume more current than others in the array. This causes unevenness in light output, and as the LEDs start to pass more current the forward voltage drop will decrease allowing more current to pass through the LED. This will continue until the LEDs that are hogging the current experience thermal runaway and are destroyed by overheating. After this occurs, excessive current is available to the remaining LEDs which quickly causes thermal runaway to incur in the remaining LEDs, destroying them as well.
While there are some conventional systems with a parallel array of LEDs, there is no solution to the light strip being flexible on more than one axis. Because of the geometry of having parallel buses of a fixed length affixed to the LEDs in conventional systems, the LED lighting device will be able to flex about one axis along its length. However, if the conventional LED lighting device is flexed about a second axis the electrical bus on the outside of the radius breaks due to the need to elongate to accommodate the increased distance as a result of the curvature. For example, if an LED light strip is arranged in such a way that its length is along the Y axis, its width is along the X axis, and its height is along the Z axis such that its total length is 200 mm and the distance between the buses is 3.5 mm, it is easily flexed about the X axis without damage to the electrical buses because the electrical buses are coplanar along the Y axis. However, if it is flexed about the Z axis with a 100 mm radius the inner bus would need to decrease and the length of the outer bus would need to increase. This is not possible in conventional systems because the buses effectively create a parallelogram and the buses are no longer coplanar. The length of the inner bus would need to decrease and the length of the outer bus would need to increase. Having two electrical buses of a fixed length electrically connected to LEDs in a parallel fashion effectively creates a light strip with a planer nature comparable to products that are manufactured on a flexible printed circuit board.
This planer nature and the ability to only flex about one axis along the length of the light strip limits the ability of the conventional lighting devices to be conformably mounted to surfaces with compound curvatures. Many automotive applications require mounting on surfaces of compound curvatures.